linux-kernel - will-it-scale.per_process_ops -15% regression on v6.16-rc1

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Message-ID: <639f17bf-8d4c-47a3-aebc-8d71597aba3c@amd.com>
Date: Thu, 10 Jul 2025 17:21:57 +0530
From: "D, Suneeth" <Suneeth.D@....com>
To: <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: will-it-scale.per_process_ops -15% regression on v6.16-rc1

Greetings,

We have observed ~(15-16)% regression on the will-it-scale-page_fault3 
variant when the testcase was run in process mode on the kernel 
v6.16-rc5. Further investigation showed that the regression was sparked 
in v6.16-rc1 and has propagated all its way through v6.16-rc5.

Below are the test parameters and Machine configuration that have been 
tested with:-

testcase: will-it-scale
compiler: gcc-13
test machine: 256 threads 1 sockets AMD EPYC 9754 128-Core Processor @ 
2.2GHz [Bergamo] with 258G memory
Test params:

         nr_task: [1 8 128 192 256]
	mode: process
	test: page_fault3
	cpufreq_governor: performance

stable version (v6.15)	%diff 	per_process_ops	kernel_rc_ver
----------------------  -----   --------------- -------------
492057			-15%	416927		v6.16-rc1
492057			-16%	414140		v6.16-rc2
492057			-15%	419158		v6.16-rc3
492057			-15%	420476		v6.16-rc4
492057			-15%	416334		v6.16-rc5

Also had a run with latest stable

v6.15	%diff 	v6.15.5
-----   -----   -------
492057	 1%	494990

Recreation steps:

1) git clone https://github.com/antonblanchard/will-it-scale.git
2) git clone https://github.com/intel/lkp-tests.git
3) cd will-it-scale && git apply 
lkp-tests/programs/will-it-scale/pkg/will-it-scale.patch
4) make
5) python3 ./runtest.py page_fault3 32 process 0 0 1 8 128 192 256

NOTE: [5] is specific to machine's architecture. starting from 1 is the 
array of no.of tasks that you'd wish to run the testcase which here is 
no.cores per CCX, per NUMA node/ per Socket, nr_threads.

Currently bisection is under progress b/w v6.15 and v6.16-rc1 to spot 
the culprit commit.

Thanks & Regards,
Suneeth D